Age-related changes in size, distribution and function of various lymphocyte populations will be made in rats with the object of relating those changes to host resistance to cancer. Purified T cells prepared by cell chromatography from the thoracic duct lymph of donor rats will be labeled radioactively and transfused into normal syngeneic recipients. Dilution of the transfused cells in the circulating T cell pool of individual recipients will be measured by a dilution technique. Among other parameters to be measured are the ability-of aging rats to generate lymphocytes which carry immunological memory and antigen- activated T cells which mediate cellular resistance to infection.